Mid Sweden University

Bark beetles have caused extensive damage to forests in central Sweden during the past decade, and the four-eyed spruce bark beetle, Polygraphus poligraphus, seems to be involved. However, its role in these bark beetle outbreaks is still not clear. The purpose of this study was to develop an efficient pheromone bait for P. poligraphus, which would make it possible to study the species more carefully and thereby contribute to protect exposed forests in an environmentally friendly way. Three field studies were conducted in 2015, 2016 and 2018 in Medelpad, county of Västernorrland, Sweden. The pheromone of P. poligraphus, (−)-terpinen-4-ol, was tested at different release rates and in different enantiomeric purities, to find the most attractive formulation for the beetles. It was also tested in combination with racemic frontalin, a compound which has previously been shown to produce a synergistic effect together with (−)-terpinen-4-ol of low enantiomeric purity; 52% ee. Other compounds, chosen based on responses from electroantennographic studies, were also tested in an attempt to find additional attractants and repellents for P. poligraphus. The most attractive treatment tested was enantiomerically pure (−)-terpinen-4-ol (99% ee). When the enantiomeric purity was lower (50% ee), the trap catches was lowered to levels comparable to the catches for unbaited control traps. A strong synergistic effect with frontalin was observed for (−)-terpinen-4-ol of low enantiomeric purity (50% ee) but not for the enantiomerically pure compound (99% ee). The release rate of (−)-terpinen-4-ol (99% ee) was shown to be an important factor. For the combination of frontalin and (−)-terpinen-4-ol (50% ee), the attraction seemed strongest when (−)-terpinen-4-ol was released at a higher rate than frontalin. An interesting and novel result was that a repellent compound, α-terpineol, was identified in our studies. Our results from field studies and electroantennography recordings also indicate that (+)-terpinen-4-ol is a repellent for P. poligraphus.

The bark beetle Polygraphus punctifrons (Coleoptera: Curculionidae) is a species that feeds on Norway spruce (Picea abies) and is found in the Northern parts of Europe and Russia. The release of volatile organic compounds (VOCs) produced by males and females of P. punctifrons when the beetles bore into spruce stem sections in a laboratory environment was studied using solid phase microextraction (SPME). The sampled VOCs emitted by boring beetles were analysed by gas chromatography and mass spectrometry (GCMS). (+)-2-[(1R,2S)-1-Methyl-2-(prop-1-en-2-yl)cyclobutyl]ethanol [(+)-(1R,2S)-grandisol] and (−)-(R)-1-isopropyl-4-methyl-3-cyclohexen-1-ol [(−)-(R)-terpinen-4-ol] were identified to be male specific volatiles. The identity of the compounds was confirmed by comparison with synthetic samples. Field trials with synthetic compounds in Sweden showed that racemic grandisol per se was strongly attractive for both males and females, while (−)-(R)-terpinen-4-ol was not. Further, when adding (−)-(R)-terpinen-4-ol to rac-grandisol, a synergistic effect was observed as the trap catch of P. punctifrons was fourfold. (−)-(R)-Terpinen-4-ol by its own did not attract P. punctifrons but Polygraphus poligraphus, and the latter was also attracted to traps baited with a 10:90 mixture of the two compounds. Thus, we have identified (+)-(1R,2S)-grandisol as a main component and (−)-(R)-terpinen-4-ol as a minor component of the aggregation pheromone of P. punctifrons. This opens future possibilities to monitor and, if necessary, manage populations of P. punctifrons. 

Bark beetles of the genus Polygraphus have recently been involved in large bark beetle outbreaks in central Sweden, together with the European spruce bark beetle Ips typographus. Three species of Polygraphus can be found in this region; Polygraphus poligraphus, Polygraphus punctifrons and Polygraphus subopacus. Efficient pheromone traps would facilitate further investigations of these species and their role in bark beetle outbreaks. Pheromone compounds have previously been identified in P. poligraphus and P. punctifrons, but not in P. subopacus. Thus, we allowed males and females of P. subopacus to bore in the bark of stem sections of Norway spruce (Picea abies) in the laboratory. Volatile organic compounds from boring insects were sampled with SPME and analysed with GC–MS and several male-specific compounds were observed. The male specific compounds were 3-methyl-3-buten-1-ol, 3-methyl-2-buten-1-ol, 3-methyl-2-butenal, grandisol, fragranol, (Z)-2-(3,3-dimethylcyclohexylidene)-ethanol, (E)-2-(3,3-dimethylcyclohexylidene)-ethanol, (Z)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, (E)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, geranial and γ-isogeraniol. (Z)-2-(3,3-dimethylcyclohexylidene)-ethanol, [(Z)-DMCHE], was identified from GC–MS analysis to be the major male-specific compound while the (E)-isomer, [(E)-DMCHE], was found as a minor compound. These two compounds gave positive responses in EAG analyses with antennae from males and females of P. subopacus. Thus, (Z)- and (E)-DMCHE were used in a field experiment in central Sweden but only (Z)-DMCHE was found to be attractive to males and females of P. subopacus. Consequently, (Z)-DMCHE was established to be a component of P. subopacus aggregation pheromone. 

Polygraphus proximus, a four-eyed fir bark beetle, is an invasive bark beetle species which has caused extensive damage to forests of Abies sibirica in southern and western Siberia and to Abies species in the European part of Russia. There is a high risk that the pest insect will spread to areas where it is currently not considered present, such as the European Union. In these areas, it threatens to attack conifer forests of various species which may result in major environmental and economic impact. The aim of this study was to identify pheromone components of P. proximus that can be used as pheromone baits. Males and females of P. proximus were allowed to bore into the bark of stem sections of Abies sibirica at the laboratory, and volatiles were collected with solid-phase microextraction (SPME). Analyses of these extracts with gas chromatography-mass spectrometry (GC–MS) revealed several sex-specific compounds. In total, twelve male-specific compounds and one female-specific compound were identified. The major male-specific compound determined by GC peak area was (Z)‐2‐(3,3‐dimethylcyclohexylidene)‐ethanol [(Z)-DMCHE] and the minor male-specific compounds were 3-methyl-3-buten-1-ol, 3-methyl-2-buten-1-ol, 3-methyl-2-butenal, benzyl alcohol, fragranol, 7-methyl-3-methylene-6-octen-1-ol, (Z)- and (E)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, geraniol, geranial and papayanol. The only female-specific compound was identified as 1-hexanol. Two of the male-specific compounds, (Z)‐DMCHE and 3-methyl-2-buten-1-ol were shown to attract males and females of P. proximus in field studies. Thus, we now for the first time can present the structures of two male-specific components that are biologically active parts of P. proximus aggregation pheromone. However, some chemical communication overlap between P. proximus and P. subopacus needs to be further investigated as (Z)‐DMCHE also attracted males and females of P. subopacus. 

Four-eyed bark beetles of the genus Polygraphus have been involved in large bark beetle outbreaks in different parts of the world, resulting in major economic losses. A striking example is the invasive species Polygraphus proximus which is a pest on Abies sibirica in Russia. In Sweden, Polygraphus poligraphus has been involved in bark beetle outbreaks on Norway spruce, Picea abies, together with the European spruce bark beetle Ips typographus. Two related species, Polygraphus punctifrons and Polygraphus subopacus are also present in Sweden. Recently, aggregation pheromones or pheromone components have been identified for these four Polygraphus species. However, questions remain regarding the complete composition of their pheromones, particularly for P. subopacus and P. proximus, whose aggregation pheromones appear to be very similar. In an attempt to better understand the chemical communication of these species, additional studies were conducted on P. poligraphus, P. punctifrons and P. subopacus using solid phase microextraction coupled with gas chromatography and mass spectrometry (SPME–GC–MS), electroantennography (EAG) as well as SPME–GC–MS and GC–MS with electroantennographic detection (EAD). Field experiments were also conducted. In P. punctifrons, some male-specific compounds were found in addition to those previously identified. In EAG and SPME–GC–MS/EAD studies, all three Polygraphus species responded strongly to grandisol. Using a chiral column, GC–MS/EAD revealed that they were able to detect both enantiomers of grandisol. In summary, this work presents our current understanding of the aggregation pheromones in four Polygraphus species and the challenges we have met in identifying species-specific pheromone blends for some of these species.